1. Field of the Invention (Technical Field):
The present invention relates to avoidance of wetstacking in internal combustion engines, especially in diesel-powered electric generators.
2. Background Art:
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Diesel engine driven electric generators sometimes operate for extended periods with little or no load. Under these conditions, the performance of the engine will deteriorate and if no intervention is taken, such as imposition of a load, it will ultimately fail and require expensive overhaul. The basic mechanism is not well understood, but unburned liquid hydrocarbons and/or soot in the exhaust or piston ring grooves are usually noted. The condition is referred to as “wetstacking”.
The term wetstacking is attached to any of several causes of deteriorating performance or failure of diesel engines following operation at unloaded/underloaded conditions for extended periods of time. It has been noted to occur principally in stand-by electric generator applications corresponding to engine operation when zero or only partial power output is needed. In a circumstance such as remote field operations, the generator may be the only source of power and it can potentially operate continuously at unloaded/underloaded conditions.
Observables regarding onset of wetstacking are not definite and well documented, so unless some record of load as a function of time is maintained, rough running or failure of the engine sometimes is the first evidence of a problem. Postmortum examination of failed engines by mechanics and engineers has provided information regarding the typical failure mechanism. One observation is that liquid hydrocarbons of unspecified origin can accumulate on the exhaust valve stem where the stem is exposed to the engine exhaust. As the valve moves up and down in the close tolerance valve guide, the liquid film becomes sticky and eventually causes the valve stem to seize in the guide. The valve is then immobile, and because of the hard linkage in the valve train, some mechanical member will bend or break. The engine is then out of service and cannot be recovered without extensive repairs.
Wetstacking also occurs when solid carbon or nonvolatile liquids accumulate along the cylinder wall or in the piston ring grooves and inhibit expansion and sealing of the ring against the cylinder wall. The cylinder can become glazed which further contributes to poor charge air compression heating and poor combustion of fuel. The performance of the engine continues to deteriorate until charge air compression heating is no longer sufficient for the engine to operate. The engine must then be disassembled and the carbon removed and the cylinders re-honed. Although these failure mechanisms are somewhat different, they are both presumed to be related to poor combustion of the fuel or high liquid carry-over into the exhaust when the engine is operating cool, i.e., unloaded/underloaded.
In order to avoid failures from either type of wetstacking, the conventional advice is that the engine should never be operated for extended periods of time at less than half-load. In order to assure that the engine operates at a load of at least 50%, artificial loads are sometimes applied. For electric generators, a resistive heater can be used for load, and the resulting heat dissipated either to air directly, or through the engine radiator. The shortcoming of this approach is that both fuel consumption and wear on the engine may be unnecessarily increased. An alternate to using the artificial load has been to monitor the carbon or unburned fuel content of the crankcase oil and when a threshold value is reached the oil is changed. This approach is premised on the concept that carbon or raw fuel on the cylinder walls and in the ring grooves can be washed down by the lubricating oil, but once the oil carbon or fuel content becomes high, the oil itself provides the mechanism whereby carbon is transported to the piston-cylinder clearance. Another method for dealing with wetstacking is to use a fuel additive that will encourage complete combustion. While some benefit was noted in this study, fuel additives are not compatible with single fuel strategies.
It is generally known that combustion processes are improved when the reaction temperature is high; conversely when low. This advice relates to pollution emissions such as carbon monoxide, unburned fuel, partially combusted or reformed hydrocarbons, and soot. Because an electric generator must operate at a constant speed in order to produce the desired electric output frequency, and because the amount of air passing through a diesel engine is only a function of speed, then low loads correspond to low fuel/air ratios. Consequently, the excess air dilutes energy released by fuel combustion with a consequential lowered product gas temperature. Diesel engines are always designed so that sufficient air is available to react completely with all of the fuel that is supplied at full load.
An excellent overview of operations contributing to wetstacking, symptoms of wetstacking and various methods for avoidance were given by Ronald A. Johnson, et al.; “The Part-Load Problem”, The Northern Engineer, Vol. 18, No. 1, pp. 4-7 (Spring 1986). K. Garrett, “Reducing diesel exhaust gas odour—an account of some work done by Man”, The South African Mechanical Engineer, Vol. 28, pp. 46-47 (February 1978) provides recommendations for reducing diesel exhaust gas odors, including intake throttling and injection air heating. The goal was to reduce exhaust odor that was attributed to incomplete combustion, rather than to eliminate wetstacking, and the method of intake air heating utilized a small combustor ahead of the intake manifold. Work by Watanabe et al., “Throttling of 2-Stroke Cycle Diesel Engines at Part-Load and Idling”, Society of Automotive Engineers International Automotive Engineering Congress (1973) related to wetstacking in a two cycle diesel engine and throttling was accomplished by bypass of the scavenge air from the blower.
In the patent literature, U.S. Pat. Nos. 4,951,460 and 5,083,423 deal with turbine engines and will not be discussed further because these engines are noted to exhibit a different manifestation of poor combustion, i.e., build-up of incomplete combustion product deposits in the combustor or turbine blades. In fact, the problem is more difficult here because turbine engines must pass massive amounts of excess air to moderate combustion temperature at full load. As with diesel engines, turbine engines pass a constant amount of air through the compressor for a given speed, but at low loads, very little fuel is burned so the combustion is ultra-cool and incomplete.
U.S. Pat. Nos. 4,207,848, 4,545,357, 4,723,527, 4,944,260, and 5,094,198 all provide various methods and controls to elevate intake air temperature. The elevation of intake air will address the basic problem of cool combustion, but the source of heat is the concern. For example, water jacket heat exchange can be used, but the temperature is thermostatically controlled to around 185 F; far too cool for sufficient intake air heating. Alternately, the exhaust is sufficiently hot for heat exchange to provide adequate intake air heating. However, soot always occurs in the exhaust of a diesel engine and this will ultimately foul the surfaces and can cause a variety of problems including possible ignition of the carbon residue, or at minimum, a degradation of the heat exchange surfaces. If the engine is driving an electric generator, a resistive heating element can be used, but this will draw additional power from the engine, with additional fuel consumption.
U.S. Pat. No. 4,020,814 actually seeks the opposite effect; that is, the limitation of the combustion temperature, which is to be accomplished by limiting fuel flow to the engine. This action does not contribute to solving the wetstacking problem.
U.S. Pat. No. 4,367,700 advocates the dedication of two cylinders of a six cylinder engine to combustion air preheat rather than power production, thereby creating a substantial derating of the engine. Because of the derating, this approach is not evaluated as feasible.
U.S. Pat. No. 4,254,752 relates to 2-cycle engines. The 2-cycle engine utilizes a mechanically driven blower in order to provide air for scavenging exhaust gases from the cylinder as ports in the cylinder wall are uncovered at around the midpoint of the piston down stroke. Because of this, the engine has much higher airflow than a naturally aspirated or even a turbocharged 4-cycle diesel engine. Consequently, for part load with full speed, a 2-cycle diesel engine passes copious volumes of excess air through the engine with limited fuel consumption, thereby exacerbating the wetstacking condition. This patent does state that the exhaust restriction method has application to 4-cycle diesel-engines, but no attempt is made to address adoption to that case. Superficially, this method is simpler and cheaper than the approach of the current patent application. In summary, the combustion temperature is made higher by the retention of an inverse fraction (to fuel flow) of the previous power stroke combustion gases via a “weighted” exhaust restricting butterfly valve. However, modification of injectors, as suggested in this patent, makes this approach considerably more expensive. The method does not provide for a minimum in combustion temperature because the restriction is not actively controlled to a parameter that is affected by the action that is taken. Further, the elevation of a weight about a pivot does not provide for a linear effect. In other words, the exerted force (and pressure) is greatest when the weight is horizontal and essentially zero when the weight is vertical. This method is in contrast to the present invention, which utilizes an observation (exhaust temperature) as a feedback that is directly related to the remedy that has been taken (restriction of excess air flow through the cylinder). Because the latter is a “smart” control, it can compensate for a number of variables, including: change of engine size, type (e.g., 2-cycle or 4 cycle) or manufacture, altitude change, intake (ambient) temperature change, engine performance degradation, etc. Expanding the comment on engine variety, the cited patent stipulates that it is applicable to multi-cylinder engines. This restriction is probably because for a single or 2-cylinder engine, there is pulsation in the exhaust flow as exhaust valves or ports are opened and closed. This pulsation would cause the exhaust restrictor valve weight to oscillate, thereby causing a feedback oscillation in all flows within the engine. Engine oscillations are generally highly undesirable. The current invention, however, is not subject to such oscillations because the controller will respond slower than the cycle time of the cylinder and will not subject the engine to pulsations. A consequence of the exhaust restriction approach is an increase in the cylinder pressure at all operating conditions. Elevated cylinder pressure can contribute to ring blow-by with dilution of crankcase oil, and exacerbate soot collection in piston ring grooves (associated with symptoms of wetstacking). And because even at full load there will always be a pressure restriction in the exhaust required to hold the weight in an elevated position, the thermal efficiency of the engine will unquestionably be compromised.
The present invention solves the wetstacking problem by raising the reaction temperature by (primarily) restricting the flow of intake air and (secondarily), if desired, elevating the temperature of the intake air, combined with “smart” control of those operations. In comparison to the case where there are no intervention measures, a given quantity of fuel releases a given amount of energy to the gas charge. If the initial temperature of the air is higher, then the final temperature of the gas after combustion will be higher. Or, if the amount of excess air in the charge is reduced, then the final temperature will be again higher. Consequently, both of these methods raise combustion temperature and either can be used as an intervention technique. For intake air heating, a heat exchanger with the engine exhaust is adequate. However, heat exchangers are somewhat expensive, comparatively large, and the exhaust side can foul, due to accumulation of soot on heat transfer surfaces. Intake air throttling can be done with a simple throttle valve that requires minimum space, is inexpensive, is reliable, and can be fully opened to return the engine to full power capability. Accordingly, the invention preferably restricts air intake according to a control mechanism, which is, if desired, supplemented by raising the temperature of intake air via water jacket heat exchange.